1. Field of the Invention
The present invention relates to a valve timing control apparatus for an internal combustion engine, the valve timing control apparatus being capable of learning a hold control amount that is used for maintaining actual valve timing at a present state.
2. Description of Related Art
A variable valve timing control apparatus is known to change opening and closing timing (valve timing) of at least one of intake and exhaust valves of an internal combustion engine (engine) in accordance with the operational state of the engine. In general, in a hydraulic variable valve timing control apparatus that uses oil pressure as a drive source, a hydraulic control valve (OCV) is controlled based on a duty cycle such that pressure of oil supplied to a variable valve timing mechanism is adjusted. As a result, a rotational phase difference between a crankshaft and a camshaft of the engine is changed, and thereby valve timing is changed accordingly.
More specifically, the hydraulic control valve is controlled based on a control duty value that is computed based on a hold duty value (hold control amount) and a deviation between target valve timing and actual valve timing. The above target valve timing is typically computed based on an operational state of the engine. The hold duty value is defined as a duty value (duty cycle) measured when a speed of change of the valve timing is zero, and thereby a hold duty value indicates the duty value that is used for holding or maintaining present valve timing.
In general, the hold duty value changes with variations in the manufacturing tolerances or the aged deterioration of a variable valve timing mechanism or a hydraulic control valve, with an oil temperature, or with an engine rotation speed. When the hold duty value changes, it is impossible to accurately control the actual valve timing, and thereby the accuracy of the variable valve timing control may deteriorate. Thus, there is needed to learn the hold duty value in accordance with the state of the engine as required. For example. JP-A-H11-36905 describes a technique, in which a control duty value is learned as the hold duty value when the following conditions are satisfied. The deviation between the target valve timing and the actual valve timing is equal to or greater than a predetermined value, and both the target valve timing and the actual valve timing have not changed for a predetermined time period.
Because a learning execution condition for executing the learning operation includes that both the target valve timing and the actual valve timing have not changed for the predetermined time period in JP-A-H11-36905 as above, it may require more time for the actual valve timing to become the target valve timing when a responsivity of the variable valve timing mechanism deteriorates for some reasons, for example. As a result, the satisfaction of the learning execution condition may be delayed, and thereby an unwanted deviation between the actual valve timing and the target valve timing may cause an insufficient torque generation disadvantageously during a period before the learning execution condition becomes satisfied. Furthermore, when the satisfaction of the learning execution condition is delayed, the frequency of satisfying the learning execution condition becomes reduced accordingly. As a result, the frequency of learning of the hold duty value may be reduced, and thereby the accuracy in the learning of the hold duty value may deteriorate disadvantageously.